[Makefile] Check for LLVM_CONFIG only when LLVM is used

[straight-shoota]

This is a re-issue of #11509 with an improved implementation of #11454 that does not use prerequisites. It adds a custom function call to every recipe that requires LLVM to be available. That function only runs once for the entire Makefile execution, so you won't get multiple Using llvm-config prints.

Additionally fixes some use cases around colorized output. This should now work with every shell and output only gets colorized if the terminal supports that (tput makes sure of that).

[j8r]

What was wrong with shell escapes? I spawned an alpine container, and tput is not pre-installed.

[straight-shoota]

It looks ugly when your terminal doesn't support escape codes.

tput not being available by default is a bummer. I was happy to have a simple solution. But I can work around it and check for TERM explicitly. I had done this in an experiment already, but tput is easier.
I suppose we could alternatively ignore missing tput. Then you only get color when it's available 🤷

[beta-ziliani]

LGTM